Multiple beam inspection apparatus and method

ABSTRACT

Disclosed is an optical inspection system for inspecting the surface of a substrate. The optical inspection system includes a light source for emitting an incident light beam along an optical axis and a first set of optical elements arranged for separating the incident light beam into a plurality of light beams, directing the plurality of light beams to intersect with the surface of the substrate, and focusing the plurality of light beams to a plurality of scanning spots on the surface of the substrate. The inspection system further includes a light detector arrangement including individual light detectors that correspond to individual ones of a plurality of reflected or transmitted light beams caused by the intersection of the plurality of light beams with the surface of the substrate. The light detectors are arranged for sensing the light intensity of either the reflected or transmitted light.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/075,634, filed Mar. 8, 2005 now U.S. Pat. No. 7,075,638, entitled“Multiple Beam Inspection Apparatus and Method”, which is a continuationof U.S. patent application Ser. No. 09/636,124, filed Aug. 10, 2000 nowU.S. Pat. No. 6,879,390, entitled “Multiple Beam Inspection Apparatusand Method” which is hereby incorporated by reference.

This application is related to U.S. Pat. No. 6,636,301, entitled“Multiple Beam Inspection Apparatus and Method”, the content of which ishereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to apparatus and methods for inspectingthe surface of a substrate such as reticles, photomasks, wafers and thelike (hereafter referred to generally as photomasks). More particularly,the present invention relates to an optical inspection system that canscan such a substrate at a high speed and with a high degree ofsensitivity.

Integrated circuits are made by photolithographic processes, which usephotomasks or reticles and an associated light source to project acircuit image onto a silicon wafer. The presence of defects on thesurfaces of the photomasks is highly undesirable and adversely affectsthe resulting circuits. The defects can be due to, but not limited to, aportion of the pattern being absent from an area where it is intended tobe present, a portion of the pattern being present in an area where itis not intended to be, chemical stains or residues from the photomaskmanufacturing processes which cause an unintended localized modificationof the light transmission property of the photomask, particulatecontaminates such as dust, resist flakes, skin flakes, erosion of thephotolithographic pattern due to electrostatic discharge, artifacts inthe photomask substrate such as pits, scratches, and striations, andlocalized light transmission errors in the substrate or pattern layer.Since it is inevitable that defects will occur, these defects have to befound and repaired prior to use. Blank substrates can also be inspectedfor defects prior to patterning.

Methods and apparatus for detecting defects have been around for sometime. For example, inspection systems and methods utilizing laser lighthave been introduced and employed to various degrees to scan the surfaceof substrates such as photomasks, reticles and wafers. These laserinspection systems and methods generally include a laser source foremitting a laser beam, optics for focussing the laser beam to a scanningspot on the surface of the substrate, a stage for providingtranslational travel, collection optics for collecting eithertransmitted and/or reflected light, detectors for detecting either thetransmitted and/or reflected light, sampling the signals at preciseintervals and using this information to construct a virtual image of thesubstrate being inspected. By way of example, representative laserinspection systems are described in U.S. Pat. No. 5,563,702 to Emery etal., U.S. Pat. No. 5,737,072 to Emery et al., U.S. Pat. No. 5,572,598 toWihl et al., and U.S. Pat. No. 6,052,478 to Wihl et al., each of whichare incorporated herein by reference.

Although such systems work well, there are continuing efforts to improvetheir design to provide greater sensitivity and faster scanning speeds.That is, as the complexity of integrated circuits has increased, so hasthe demand on the inspection process. Both the need for resolvingsmaller defects and for inspecting larger areas have resulted in muchgreater magnification requirements and in much greater speedrequirements, for example, in terms of number of pixels (pictureelements) per second processed.

In view of the foregoing, there is a need for improved inspectiontechniques that provide increased scanning speeds.

SUMMARY OF THE INVENTION

Accordingly, the present invention addresses some of the above problemsby providing improved apparatus and methods for performing aninspection. In general terms, the inspection system includes componentsarranged to generate a plurality of beams incident on a sample, such asa photomask. The inspection system also includes components fordetecting a plurality of beams that are reflected or transmitted fromthe sample as a result of the incident beams.

In one embodiment, an optical inspection system for inspecting thesurface of a substrate is disclosed. The optical inspection systemincludes a light source for emitting an incident light beam along anoptical axis and a first set of optical elements arranged for separatingthe incident light beam into a plurality of light beams, directing theplurality of light beams to intersect with the surface of the substrate,and focusing the plurality of light beams to a plurality of scanningspots on the surface of the substrate. The inspection system furtherincludes a light detector arrangement including individual lightdetectors that correspond to individual ones of a plurality of reflectedor transmitted light beams caused by the intersection of the pluralityof light beams with the surface of the substrate. The light detectorsare arranged for sensing the light intensity of either the reflected ortransmitted light.

In a specific implementation, the first set of optical elements isarranged for separating the incident light beam into a plurality ofspatially distinct light beams, which are offset and staggered relativeto one another. In a more specific implementation, the plurality ofspatially distinct light beams consist of a first light beam, a secondlight beam and a third light beam.

In a preferred embodiment, the inspection system further includes asecond set of optical elements adapted for collecting either a pluralityof reflected light beams or a plurality of transmitted light beamscaused by the intersection of the plurality of light beams with thesurface of the substrate. The second set of optical elements is arrangedfor collecting the plurality of spatially distinct light beams, whichhave intersected with the surface of the substrate, and for directingindividual ones of the collected light beams to individual lightdetectors of the light detector arrangement.

In one implementation, the first set of optical elements includes a beamdeflector disposed along the first optical axis. The beam deflector isarranged for deflecting the light beam such that the scanning spots arecaused to sweep across the surface of the substrate in substantially onedirection from a first point to a second point. In a specificembodiment, the beam deflector includes an acousto-optic device forcausing the light beam to be deflected over a relatively small angle.The angle is at least one of the factors for determining the scan lengthof each of the scanning spots. Preferably, the first set of opticalelements is formed from a beam separator disposed along the firstoptical axis, and the beam separator is arranged for separating thelight beam into the plurality of light beams. In a specificimplementation, the beam separator is a diffraction grating.

In another aspect, the invention pertains to a method of inspecting asurface of a substrate. The substrate is transported in a firstdirection, and a first light beam is provided. The first light beam isseparated into a plurality of light beams. The plurality of light beamsare focussed to a plurality of spatially distinct spots on the surfaceof the substrate. The plurality of light beams are swept so as to movethe plurality of spatially distinct spots along the surface of thesubstrate in a second direction. The intensity of each of the pluralityof light beams is detected after their intersection with the surface ofthe substrate. A plurality of scan signals corresponding to the detectedplurality of light beams are generated.

In yet another embodiment, an optical inspection system for inspecting asurface of a substrate is disclosed. The inspection system has a lightsource for emitting a light beam along an optical axis and a diffractiongrating disposed along the optical axis. The diffraction grating isarranged for separating the light beam into a plurality of light beamswhich are used to form scanning spots on the surface of the substrate.Each of the scanning spots has a specified overlap and separation withrespect to one another that is controlled by the grating spacing and therotation of the diffraction grating about the optical axis.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIG. 1 is a simplified block diagram of an optical inspection system, inaccordance with one embodiment of the present invention.

FIG. 2 is a detailed block diagram of an optical inspection system forinspecting the surface of a substrate, in accordance with one embodimentof the present invention.

FIGS. 3A-C are side view illustrations of the light source and theinspection optics of FIG. 3, as light beams are scanned across thesurface of the substrate, in accordance with one embodiment of thepresent invention.

FIGS. 4A-C are top view illustrations of the scanning spots produced bythe inspection optics of FIGS. 3A-C, as light beams are scanned acrossthe surface of the substrate, in accordance with one embodiment of thepresent invention.

FIG. 5 is a detailed top view diagram of the scanning spot distributionproduced by the inspection optics of FIG. 3A-C, in accordance with oneembodiment of the present invention.

FIG. 6 is a side view diagram, in cross section, of a diffractiongrating, in accordance with one embodiment of the present invention.

FIGS. 7A-B are top view diagrams of a diffraction grating, in accordancewith one embodiment of the present invention.

FIG. 8 is a top view illustration of a scanning spot distribution as itis moved over a substrate, in accordance with one embodiment of thepresent invention.

FIG. 9 is a side view illustration of the transmitted light optics andthe transmitted light detector arrangement of FIG. 3, in accordance withone embodiment of the present invention.

FIG. 10 is a detailed perspective diagram of a prism that can be used ineither the transmitted light optics or reflected light optics, inaccordance with one embodiment of the present invention.

FIG. 11 is a side view diagram, in cross section, of a prism that can beused in either the transmitted light optics or reflected light optics,in accordance with one embodiment of the present invention.

FIGS. 12A-C is a side view diagram illustrating the various positions ofthe first transmitted lens and the objective lens, in accordance withone embodiment of the present invention.

FIG. 13 is a flow diagram of an inspection set-up procedure, inaccordance with one embodiment of the present invention.

FIG. 14 is a side view diagram of the reflected light optics andreflected light detector arrangement of FIG. 3, in accordance with oneembodiment of the present invention.

FIG. 15 is a side view diagram of a beam separator utilizing a beamsplitter cube, in accordance to an alternate embodiment of the presentinvention.

FIG. 16 is a side view diagram of a beam separator utilizing a beamsplitter cube, in accordance to an alternate embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described in detail with reference toa few specific embodiments thereof and as illustrated in theaccompanying drawings. In the following description, numerous specificdetails are set forth in order to provide a thorough understanding ofthe present invention. It will be apparent, however, to one skilled inthe art, that the present invention may be practiced without some or allof these specific details. In other instances, well known process stepshave not been described in detail in order not to unnecessarily obscurethe present invention.

The invention pertains to an optical inspection system for inspectingthe surface of a substrate (or sample), and more particularly to anoptical inspection system that can scan a substrate at a high speed andwith a high degree of sensitivity. One aspect of the invention relatesto increasing the number of scanning spots used to inspect thesubstrate. Another aspect of the invention relates to spatiallyseparating each of the scanning spots used to inspect the substrate. Yetanother aspect of the invention relates to collecting reflected and/ortransmitted light caused by the intersection of scanning spots with thesurface of the substrate. The invention is particularly useful forinspecting substrates, such as reticles and photomasks. Furthermore, theinvention may be used to detect defects as well as to measuresemiconductor device characteristics, such as line widths.

FIG. 1 is a simplified block diagram of an optical inspection system 10,in accordance with one embodiment of the present invention. The opticalinspection system 10 is arranged for inspecting a surface 11 of asubstrate 12. The dimensions of various components are exaggerated tobetter illustrate the optical components of this embodiment. As shown,the optical inspection system 10 includes an optical assembly 14, astage 16, and a control system 17. The optical assembly 14 generallyincludes at least a first optical arrangement 22 and a second opticalarrangement 24. In general terms, the first optical arrangement 22generates two or more beams incident on the substrate, and the secondoptical arrangement 24 detects two or more beams emanating from thesample as a result of the two or more incident beams. The first andsecond optical arrangement may be arranged in suitable manner inrelation to each other. For example, the second optical arrangement 24and the first optical arrangement 22 may both be arranged over thesubstrate surface 11 so that reflected beams resulting from incidentbeams generated by the first optical arrangement 22 may be detected bythe second optical arrangement 24.

In the illustrated embodiment, the first optical arrangement 22 isarranged for generating a plurality of scanning spots (not shown) alongan optical axis 20. As should be appreciated, the scanning spots areused to scan the surface 11 of the substrate 12. On the other hand, thesecond optical arrangement 24 is arranged for collecting transmittedand/or reflected light that is produced by moving the scanning spotsacross the surface 11 of the substrate 12.

To elaborate further, the first optical arrangement 22 includes at leasta light source 26 for emitting a light beam 34 and a first set ofoptical elements 28. The first set of optical elements 28 may bearranged to provide one or more optical capabilities including, but notlimited to, separating the light beam 34 into a plurality of incidentlight beams 36, directing the plurality of incident light beams 36 tointersect with the surface 11 of the substrate 12, and focusing theplurality of incident light beams 36 to a plurality of scanning spots(not shown in FIG. 1) on the surface 11 of the substrate 12. The amountof first beams produced generally corresponds to the desired inspectionspeed. In one embodiment, the optical elements are arranged to separatethe beam 34 into three incident light beams 36. By triplicating thebeam, a wider scan is produced and therefore the resulting inspectionspeed is about three times faster than the speed produced for anon-triplicated single beam. Although only three light beams are shown,it should be understood that the number of separated light beams mayvary according to the specific needs of each optical inspection system.For example, two beams may be used or four or more beams may be used. Itshould be noted, however, that the complexity of the optic elements isdirectly proportional to the number of beams produced.

Furthermore, the second optical arrangement 24 includes at least asecond set of optical elements 30 and a light detecting arrangement 32.The second set of optical elements 30 are in the path of a plurality ofcollected light beams 40, which are formed after the plurality ofincident light beams 36 intersect with the surface 11 of the substrate12. The plurality of collected light beams 40 may result fromtransmitted light that passes through the substrate 12 and/or reflectedlight that is reflected off the surface 11 of the substrate 12. Thesecond set of optical elements 30 are adapted for collecting theplurality of collected light beams 40 and for focusing the collectedlight beams 40 on the light detecting arrangement 32. The lightdetecting arrangement 32 is arranged for detecting the light intensityof the collected light beams 40, and more particularly for detectingchanges in the intensity of light caused by the intersection of theplurality of incident light beams with the substrate. The lightdetecting arrangement 32 generally includes individual light detectors42 that correspond to each of the second light beams 40. Furthermore,each of the detectors 42 is arranged for detecting the light intensityand for generating signals based on the detected light.

With regards to the stage 16, the stage 16 is arranged for moving thesubstrate 12 within a single plane (e.g., x & y directions) and relativeto the optical axis 20, so that all or any selected part of thesubstrate surface 11 may be inspected by the scanning spots. In mostembodiments, the stage 16 is arranged to move in a serpentine fashion.With regards to the control system 17, the control system 17 generallyincludes a control computer 18 and an electronic subsystem 19. Althoughnot shown, the control system 17 may also include a keyboard foraccepting operator inputs, a monitor for providing visual displays ofthe inspected substrate (e.g., defects), a database for storingreference information, and a recorder for recording the location ofdefects. As shown, the control computer 18 is coupled to the electronicsubsystem 19 and the electronic subsystem 19 is coupled to variouscomponents of the optical inspection system 10, and more particularly tothe stage 16 and the optical assembly 14 including the first opticalarrangement 22 and the second optical arrangement 24. The controlcomputer 18 may be arranged to act as an operator console and mastercontroller of the system. That is, all system interfaces with anoperator and the user's facilities may be made through the controlcomputer 18. Commands may be issued to and status may be monitored fromall other subsystems so as to facilitate completion of operator assignedtasks.

On the other hand, the electronics subsystem 19 may also be configuredto interpret and execute the commands issued by control computer 18. Theconfiguration may include capabilities for, but not limited to,digitizing the input from detectors, compensating these readings forvariations in the incident light intensity, constructing a virtual imageof the substrate surface based on the detected signals, detectingdefects in the image and transferring the defect data to the controlcomputer 18, accumulating the output of the interferometers used totrack the stage 16, providing the drive for linear motors that move thestage 16 or components of the optical assembly 14, and monitoringsensors which indicate status. Control systems and stages are well knownin the art and for the sake of brevity will not be discussed in greaterdetail. By way of example, a representative stage, as well as arepresentative controller may be found in U.S. Pat. No. 5,563,702, whichis herein incorporated by reference. It should be understood, however,that this is not a limitation and that other suitable stages and controlsystems may be used.

As should be appreciated, the optical inspection system 10 can bearranged to perform several types of inspection, for example,transmitted light inspection, reflected light inspection andsimultaneous reflected and transmitted inspection. In transmitted lightinspection, light is incident on the substrate, a photomask for example,and the amount of light transmitted through the mask is detected. Inreflected light inspection, the light reflecting from a surface of thesubstrate under test is measured. In addition to these defect detectionoperations, the system is also capable of performing line widthmeasurement.

In most of the defect detection operations a comparison is made betweentwo images. By way of example, the comparison may be implemented by theelectronic subsystem 19 of FIG. 1. Broadly speaking, the detectors 42generate scan signals, which are based on the measured light intensity,and send the scan signals to the electronic subsystem 19. The electronicsubsystem 19, after receiving the scan signals, correspondingly comparesthe scan signals with reference signals, which are either stored in adatabase or determined in a current or previous scan.

More specifically, in die-to-die inspection mode, two areas of thesubstrate having identical features (dice) are compared with respect toeach other and any substantial discrepancy is flagged as a defect. Inthe die-to-database inspection mode, a defect is detected by comparingthe die under test with corresponding graphics information obtained froma computer aided database system from which the die was derived. Inother defect detection operations, a comparison is made between twodifferent modes of inspection. For example, in simultaneous reflectedand transmitted inspection, a comparison is made between the light thatis reflected off the surface of the substrate and light that istransmitted through the substrate. In this type of inspection theoptical inspection system performs all of the inspection tasks usingonly the substrate to be inspected. That is, no comparisons are madebetween an adjacent die or a database.

FIG. 2 is a detailed block diagram of an optical assembly 50 forinspecting the surface 11 of the substrate 12, in accordance with oneembodiment of the present invention. By way of example, the opticalassembly 50 may be the optical assembly 14 as described in FIG. 1. Theoptical assembly 50 generally includes a first optical arrangement 51and a second optical arrangement 57, both of which may respectivelycorrespond to the first optical arrangement 22 and the second opticalarrangement 24 of FIG. 1. As shown, the first optical arrangement 51includes at least a light source 52, inspection optics 54, and referenceoptics 56, while the second optical arrangement 57 includes at leasttransmitted light optics 58, transmitted light detectors 60, reflectedlight optics 62, and reflected light detectors 64.

The light source 52 is arranged for emitting a light beam 66 along afirst path 68. The light beam 66 emitted by the light source 52, firstpasses through an acousto optic device 70, which is arranged fordeflecting and focussing the light beam. Although not shown, the acoustooptic device 70 may include a pair of acousto-optic elements, which maybe an acousto-optic prescanner and an acousto-optic scanner. These twoelements deflect the light beam in the Y-direction and focus it in theZ-direction. By way of example, most acousto-optic devices operate bysending an RF signal to quartz or a crystal such as TeO₂. The signalcauses a sound wave to travel through the crystal. Because of thetravelling sound wave, the crystal becomes asymmetric, which causes theindex of refraction to change throughout the crystal. This change causesincident beams to form a focused travelling spot which is deflected inan oscillatory fashion.

When the light beam 66 emerges from the acousto-optic device 70, it thenpasses through a pair of quarter wave plates 72 and a relay lens 74. Therelay lens 74 is arranged to collimate the light beam 66. The collimatedlight beam 66 then continues on its path until it reaches a diffractiongrating 76. The diffraction grating 76 is arranged for flaring out thelight beam 66, and more particularly for separating the light beam 66into three distinct beams, which are designated 78A, 78B and 78C. Inother words, each of the beams are spatially distinguishable from oneanother (i.e., spatially distinct). In most cases, the spatiallydistinct beams 78A, 78B and 78C are also arranged to be equally spacedapart and have substantially equal light intensities.

Upon leaving the diffraction grating 76, the three beams 78A, 78B and78C pass through an aperture 80 and then continue along path 68 untilthey reach a beam splitter cube 82. The beam splitter cube 82 (workingwith the quarter wave plates 72) is arranged to divide the beams intopaths 84 and 86. Path 84 is used to distribute a first light portion ofthe beams to the substrate 12 and path 86 is used to distribute a secondlight portion of the beams to the reference optics 56. In mostembodiments, most of the light is distributed to the substrate 12 alongpath 84 and a small percentage of the light is distributed to thereference optics 56 along path 86. It should be understood, however,that the percentage ratios may vary according to the specific design ofeach optical inspection system. In brief, the reference optics 56include a reference collection lens 114 and a reference detector 116.The reference collection lens 114 is arranged to collect and direct thesecond portion of the beams, now designated 115A-C, on the referencedetector 116. As should be appreciated, the reference detector 116 isarranged to measure the intensity of the light. Although not shown inFIG. 2, the reference detector 116 is generally coupled to an electronicsubsystem such as the electronic subsystem 19 of FIG. 1 such that thedata collected by the detector can be transferred to the control systemfor analysis. Reference optics are generally well known in the art andfor the sake of brevity will not be discussed in detail.

The three beams 78A, 78B and 78C continuing along path 84 are receivedby a telescope 88. Although not shown, inside the telescope 88 there area several lens elements that redirect and expand the light. In oneembodiment, the telescope is part of a telescope system that includes aplurality of telescopes rotating on a turret. For example, threetelescopes may be used. The purpose of these telescopes is to vary thesize of the scanning spot on the substrate and thereby allow selectionof the minimum detectable defect size. More particularly, each of thetelescopes generally represents a different pixel size. As such, onetelescope may generate a larger spot size making the inspection fasterand less sensitive (e.g., low resolution), while another telescope maygenerate a smaller spot size making inspection slower and more sensitive(e.g., high resolution).

From the telescope 88, the beams 78A, 78B and 78C pass through anobjective lens 90, which is arranged for focussing the beams 78A, 78Band 78C onto the surface 11 of the substrate 12. As the beams 78A-Cintersect the surface 11 of the substrate 12 both reflected light beams92A, 92B, and 92C and transmitted light beams 94A, 94B, and 94C may begenerated. The transmitted light beams 94A, 94B, and 94C pass throughthe substrate 12, while the reflected light beams 92A, 92B, and 92Creflect off the surface 11 of the substrate 12. By way of example, thereflected light beams 92A, 92B, and 92C may reflect off of an opaquesurfaces of the substrate, and the transmitted light beams 94A, 94B, and94C may transmit through transparent areas of the substrate. Thetransmitted light beams 94 are collected by the transmitted light optics58 and the reflected light beams 92 are collected by the reflected lightoptics 62.

With regards to the transmitted light optics 58, the transmitted lightbeams 94A, 94B, 94C, after passing through the substrate 12, arecollected by a first transmitted lens 96 and focussed with the aid of aspherical aberration corrector lens 98 onto a transmitted prism 100. Asshown, the prism 100 has a facet for each of the transmitted light beams94A, 94B, 94C that are arranged for repositioning and bending thetransmitted light beams 94A, 94B, 94C. In most cases, the prism 100 isused to separate the beams so that they each fall on a single detectorin the transmitted light detector arrangement 60. As shown, thetransmitted light detector arrangement 60 includes three distinctdetectors 61A-C, and more particularly a first transmission detector61A, a second transmission detector 61B, and a third transmissiondetector 61C. Accordingly, when the beams 94A-C leave the prism 100 theypass through a second transmitted lens 102, which individually focuseseach of the separated beams 94A, 94B, 94C onto one of these detectors61A-C. For example, beam 94A is focused onto transmission detector 61A;beam 94B is focused onto transmission detector 61B; and beam 94C isfocused onto transmission detector 61C. As should be appreciated, eachof the transmission detectors 61A, 61B, or 61C is arranged for measuringthe intensity of the transmitted light.

With regards to the reflected light optics 62, the reflected light beams92A, 92B, and 92C after reflecting off of the substrate 12 are collectedby the objective lens 90, which then directs the beams 92A-C towards thetelescope 88. Before reaching the telescope 88, the beams 92A-C alsopass through a quarter wave plate 104. In general terms, the objectivelens 90 and the telescope 88 manipulate the collected beams in a mannerthat is optically reverse in relation to how the incident beams aremanipulated. That is, the objective lens 90 re-collimates the beams 92A,92B, and 92C, and the telescope 88 reduces their size. When the beams92A, 92B, and 92C leave the telescope 88, they continue along path 84(backwards) until they reach the beam splitter cube 82. The beamsplitter 82 is arranged to work with the quarter wave-plate 104 todirect the beams 92A, 92B, and 92C onto a path 106.

The beams 92A, 92B, and 92C continuing on path 106 are then collected bya first reflected lens 108, which focuses each of the beams 92A, 92B,and 92C onto a reflected prism 110, which includes a facet for each ofthe reflected light beams 92A-C. The reflected prism 110 is arranged forrepositioning and bending the reflected light beams 92A, 92B, 92C.Similar to the transmitted prism 100, the reflected prism 110 is used toseparate the beams so that they each fall on a single detector in thereflected light detector arrangement 64. As shown, the reflected lightdetector arrangement 64 includes three individually distinct detectors65A-C, and more particularly a first reflected detector 65A, a secondreflected detector 65B, and a third reflected detector 65C. Of course,each detector may be packaged separately or together. When the beams92A-C leave the prism 110, they pass through a second reflected lens112, which individually focuses each of the separated beams 92A, 92B,92C onto one of these detectors 65A-C. For example, beam 92A is focusedonto reflected detector 65A; beam 92B is focused onto reflected detector65B; and beam 92C is focused onto reflected detector 65C. As should beappreciated, each of the reflected detectors 65A, 65B, or 65C isarranged for measuring the intensity of the reflected light.

There are a multiplicity of inspection modes that can be facilitated bythe aforementioned optical assembly 50. By way of example, the opticalassembly 50 can facilitate a transmitted light inspection mode, areflected light inspection mode, and a simultaneous inspection mode.With regards to transmitted light inspection mode, transmission modedetection is typically used for defect detection on substrates such asconventional optical masks having transparent areas and opaque areas. Asthe light beams 94A-C scan the mask (or substrate 12), the lightpenetrates the mask at transparent points and is detected by thetransmitted light detectors 61A-C, which are located behind the mask andwhich measure the light of each of the light beams 94A-C collected bythe transmitted light optics 58 including the first transmitted lens 96,the second transmitted lens 102, the spherical aberration lens 98, andthe prism 100.

With regards to reflected light inspection mode, reflected lightinspection can be performed on transparent or opaque substrates thatcontain image information in the form of chromium, developed photoresistor other features. Light reflected by the substrate 12 passes backwardsalong the same optical path as the inspection optics 54 but is thendiverted by a polarizing beam splitter 82 into detectors 65A-C. Moreparticularly, the first reflected lens 108, the prism 110 and the secondreflected lens 112 project the light from the diverted light beams 92A-Conto the detectors 65A-C. Reflected light inspection may also be used todetect contamination on top of opaque substrate surfaces.

With regards to simultaneous inspection mode, both transmitted light andreflected light are utilized to determine the existence and/or type of adefect. The two measured values of the system are the intensity of thelight beams 94A-C transmitted through the substrate 12 as sensed bytransmitted light detectors 61A-C and the intensity of the reflectedlight beams 92A-C as detected by reflected light detectors 65A-C. Thosetwo measured values can then be processed to determine the type ofdefect, if any, at a corresponding point on the substrate 12.

More particularly, simultaneous transmitted and reflected detection candisclose the existence of an opaque defect sensed by the transmitteddetectors while the output of the reflected detectors can be used todisclose the type of defect. As an example, either a chrome dot or aparticle on a substrate may both result in a low transmitted lightindication from the transmission detectors, but a reflective chromedefect may result in a high reflected light indication and a particlemay result in a lower reflected light indication from the same reflectedlight detectors. Accordingly, by using both reflected and transmitteddetection one may locate a particle on top of chrome geometry whichcould not be done if only the reflected or transmitted characteristicsof the defect was examined. In addition, one may determine signaturesfor certain types of defects, such as the ratio of their reflected andtransmitted light intensities. This information can then be used toautomatically classify defects.

By way of example, representative inspection modes including, reflected,transmitted and simultaneous reflected and transmitted modes, may befound in U.S. Pat. No. 5,563,702, which is herein incorporated byreference. It should be understood, however, that these modes are not alimitation and that other suitable modes may be used.

Referring now to FIGS. 3-8, the inspection optics 54 will be describedin greater detail. In brief, FIGS. 3A-C are side view illustrations ofthe light source 52 and the inspection optics 54, as the light beams78A-C are scanned across the surface 11 of the substrate 12, inaccordance with one embodiment of the present invention. FIGS. 4A-C aretop view illustrations of the scanning spots 124A-C produced by theinspection optics 54, also as the light beams 78A-C are scanned acrossthe surface 11 of the substrate 12, in accordance with one embodiment ofthe present invention. Accordingly, FIG. 3A is associated with FIG. 4A;FIG. 3B is associated with FIG. 4B; and FIG. 3C is associated with FIG.4C. Furthermore, FIG. 5 is a detailed top view diagram of the scanningspot distribution 122 produced by the inspection optics 54, inaccordance with one embodiment of the present invention. FIG. 6 is aside view diagram, in cross section, of the diffraction grating 76, inaccordance with one embodiment of the present invention. FIGS. 7A-B aretop view diagrams of the diffraction grating 76, in accordance with oneembodiment of the present invention. FIG. 8 is a top view illustrationof the scanning spots 124A-C as they are moved across the substrate 12,in accordance with one embodiment of the present invention.

As shown in FIGS. 3 and 4, the light source 52 is configured to generatea single light beam 66 and the inspection optics 54 are configured toperform a variety of tasks associated with the single light beam 66including, but not limited to, separating the single beam of light 66into a plurality of light beams 78A-C, causing the plurality of lightbeams 78A-C to be deflected or swept over a small angle from one side(as shown in FIG. 3A) to the opposite side (as shown in FIG. 3C) of anoptical axis 120, and focussing the deflected light beams 78A-C toscanning spots 124A-C on the surface 11 of the substrate 12 (as shown inFIGS. 4A-C). As should be appreciated, scanning spot 124A corresponds tolight beam 78A; scanning spot 124B corresponds to light beam 78B; andscanning spot 124C corresponds to light beam 78C.

As shown in FIGS. 3A-C, the inspection optics 54 used to form thescanning spots 124A-C include an acousto-optic device 70, a relay lens74, a diffraction grating 76, an aperture 80, a telescope 88, and anobjective lens 90. As such, the light source 52 generates a light beam66, which is made incident on the acousto-optic device 70. Theacousto-optic device 70, in turn, deflects the light beam 66 in theY-direction and relative to the optical axis 120. For ease ofdiscussion, the deflected light beam is designated 66′. To elaboratefurther, the acousto-optic device 70 moves the beam 66 between referencepoint 126 and reference point 128. FIG. 3A shows the beam 66 after ithas been deflected to reference point 126; FIG. 3B shows the beam 66after it has been deflected to reference point 127 (which is betweenreference points 126 and 128); and FIG. 3C shows the beam 66 after ithas been deflected to reference point 128. As should be appreciated,each of these sweep positions occurs at a different time in a givensweep. For example, FIG. 3A may show the beam at a first time, T₁; FIG.3B may show the beam at a second time, T₂; and FIG. 3C may show the beamat a third time, T₃. Although, the sweeps are shown segmented in FIGS.3A-C, it should be noted that the acousto-optic device typicallyoperates at the speed of sound and as a result the sweep is almostinstantaneous.

Furthermore, the deflected distance D, defined by reference points 126and 128, generally corresponds to a scanning length L for each of thescanning spots 124A-C (as shown in FIGS. 4A & C). Moreover, thecombination of each of these individual scans or stripes (e.g., distanceL) results in a scanning swath S, which is the overall length of thethree scans. As should be appreciated, the scanning swath S is aboutthree times as large as an individual scan L, and therefore the scanningspeed is about three times as fast. That is, the field size is largerand therefore more of the substrate is inspected for each scan, i.e.,fewer turnarounds are needed with respect to the serpentine motion ofthe stage.

As shown in FIG. 3A-C, when the deflected beam 66′ emerges from theacousto-optic device 70 it is diverging towards the relay lens 74. Asthe deflected beam 66′ passes through the relay lens 74, it iscollimated such that diverging beam is turned into a collimated orparallel beam 66″. Upon leaving the relay lens 74, the collimated beam66″ is made incident on the diffraction grating 76. The diffractiongrating 76 correspondingly separates the collimated beam 66″ into threedistinct light beams 78A-C that are used to produce the scanning spotdistribution 122 (e.g., spots 124A-C). The three beams 78A-C generallymove together as a group when the acousto-optic device 70 deflects thelight beam 66 between the reference points 126 and 128.

Referring to FIGS. 6 and 7, the diffraction grating 76 is generallyformed by ruling equally spaced parallel lines or gratings 140 on aplate 142, which is formed from either glass or metal. Gratings ruled onmetal plates are called reflective type gratings because their effectsare viewed in reflected light rather than transmitted light. Conversely,gratings ruled on glass plates are called transmission type gratingsbecause their effects are viewed in transmitted light rather thanreflected light. By way of example, transmission type gratings includephase diffraction gratings, which have gratings etched into the glassplate and amplitude diffraction gratings, which have chrome gratingsdeposited on the glass plate. In phase gratings, the entire grating isformed of glass or quartz material, and therefore more light istransmitted through the grating. In amplitude gratings, the grating isformed from chrome, and therefore less light is transmitted through thediffraction grating, i.e., a portion of the light is blocked by thechrome gratings. In the illustrated embodiment, a phase diffractiongrating is used over amplitude diffraction gratings because of itsincreased efficiency in transferring light. Although a phase diffractiongrating is shown, it should be appreciated that both transmission typegratings (including amplitude gratings) and reflective type gratings maybe used in the inspection optics. Furthermore, although only a singlelevel grating is shown, it should be appreciated that multilevelgratings may also be used. In multilevel gratings, the rulings arestepped in levels with different depths.

To elaborate further, the diffraction grating 76 is arranged to split orscatter a single light beam into various diffraction orders and isgenerally controlled by the equation 2d sin θ=mλ, where d is the gratingspacing (i.e., the distance between rulings), m is the diffraction order(in integers), θ is the angle between diffraction orders, and λ is thewavelength of the light incident on the grating. As shown in FIG. 6, theincident light beam 66″ is split into diffraction orders m=1, m=0, m=−1in order to generate the three distinct beams 78A-C. Furthermore, thedepth, D, of the gratings is controlled to equalize the light beamslight level so as to produce three equally intense light beams, i.e.,the depth effects the efficiency of the transmitted light. It should benoted that the m=0 beam scans the same angular rate as the incidentbeam, but the m=1 and m=−1 beams scan at different and non-linear ratesrelative to the incident beam due to the sine term in the equation.

Referring back to FIGS. 3 and 4, each of the light beams 78A-C passesthrough the aperture 80 and is individually incident on the telescope 88upon leaving the diffraction grating 76. The aperture 80 may be a metalplate with an opening for uniformly defining the image of the lightbeams. By way of example, the opening may be a circular opening that isarranged to change a square patch of light, which may be light travelingfrom the acousto-optic device 70 and relay lens 74, into a circularpatch of light leaving the aperture 80. Furthermore, the telescope 88serves to redirect and expand the light so as to result in more precisescanning spots 124A-C. Telescopes are generally well known in the artand for the sake of brevity will not be described in greater detailherein.

When the light beams 78A-C emerge from the telescope 88, they areindividually incident on the objective lens 90. As shown in FIG. 3A-C,the light beams 78A-C are referred to as 78A′-C′ after leaving thetelescope 88. Moreover, when the light beams 78A′-C′ travel between thetelescope 88 and the objective lens 90 they pass through a pupil plane130. By definition, a pupil plane is an image of the aperture 80. Asshown in FIGS. 3A-C, the pupil plane 130 is the point where the beams78A′-C′ cross. Upon leaving the objective lens 90, each of the beams78A′-C′ is individually incident, and more particularly individuallyfocussed on the surface 11 of the substrate 12. For ease of discussion,the light beams that have passed through the objective lens 90, andwhich are focussed, are referred to as 78A″-C″. As a result of focussingthe light beams 78A″-C″ on the substrate 12, three spatially distinctscanning spots 124A-C are created on the substrate surface 11. Thespatial separation of the spots 124A-C helps to ensure the propercollection of light at the detectors.

Although not shown, the objective lens 90 is typically connected to anAuto-Focus subsystem that is arranged for maintaining the focus of thebeams as the beams are passed along the surface of the substrate. TheAuto-Focus subsystem generally includes a servo or stepper motor thatmoves the objective lens up and down along the Z axis and relative tothe substrate. As such, during the auto focus mode, the objective lensis arranged to move up and down to track the surface of the substrate soas to maintain focus. As should be appreciated, the motor is generallycontrolled by an electronic subsystem such as the electronic subsystemin FIG. 1. By way of example, a representative Auto-Focus subsystem maybe found in U.S. Pat. No. 5,563,702, which is herein incorporated byreference. It should be understood, however, that this subsystem is nota limitation and that other suitable subsystems may be used.

In addition, the diffraction grating 76 may be configured to move in andout of the optical path 120 so as to allow the single beam 66 tocontinue through the inspection optics 54 without being separated into aplurality of beams. This may be desired for alignment and set-up modes,which are used to align the optics during an inspection set-up. This mayalso be desired for viewing the substrate in a live or review mode. Thereview mode is often used to characterize a known defect. Although notshown in FIG. 3, the diffraction grating may be coupled to a drive thatis arranged for moving the diffraction grating in and out of the opticalpath. In most cases, the drive moves the diffraction grating orthogonalto the optical axis 120, for example, in the X or Y directions. By wayof example, the drive may be a stepper motor. In order to control thedrive, the drive is typically coupled to an electronic subsystem such asthe electronic subsystem 19 of FIG. 1.

Referring now to FIG. 5, the scanning spot distribution 122 will bedescribed in greater detail. As shown, the scanning spot distribution122 includes offset and staggered scanning spots 124A-C, each of whichhas a scanning length L that forms a scanning stripe 125A-C in theY-direction. Correspondingly, the combination of each of the scanningstripes 125A-C forms an overall scanning swath, S. Furthermore, thescanning spots 124A-C are spatially separated relative to one another toisolate the different scans or stripes 125A-C. By spatial separation, itis meant that each of the spots are isolated from other spots in thedistribution such that they do not cover each other during a scan. Asshould be appreciated, it would be difficult to collect the light on thethree individual detectors if the spots were not isolated in thismanner.

To elaborate further, the scanning spots 124A-C are offset (orstaggered) in both the X and Y directions. In the Y direction, the spotsare offset by approximately the distance L, and more particularly thedistance L less an overlap portion O. The overlap portion, O, is used toensure that the scanning swath, S, is scanned without missing any areastherebetween. In the X direction, the spots are offset by a distance X.The distance L, O and X may vary according to the specific needs of eachinspection system.

In accordance with one embodiment of the present invention, andreferring to FIGS. 7-9, the diffraction grating 76 is arranged tocontrol both the overlap, O, and separation, X, of the scanning spots124A-C. This can be accomplished by adjusting the grating spacing, d (asshown in FIGS. 6 & 7), and the rotation R of the diffraction grating 76about the optical axis 120 (as shown in FIGS. 7A & B). FIG. 7A shows thediffraction grating before rotation and FIG. 7B shows the diffractiongrating after rotation. As should be appreciated, both the gratingspacing, d, and the rotation, R, effect the position of the scanningspots 124A-C. The grating spacing, d, causes separation between spots,while the rotation, R, causes the outside scanning spots to be rotatedabout the optical axis 120 such that they are spatially separated inboth the X and Y directions (shown in FIG. 5 as R′). For the most part,a smaller grating spacing, d, generally produces a larger separation.Furthermore, the spots rotate about the axis (and about the center spot)at about the same angle as the diffraction grating is rotated about theaxis. It is generally believed that if the diffraction grating is notrotated, then the scanning spots, although separated, may be too closeto detect at the detectors.

Referring now to FIG. 8, a top view of the scanning spots 124A-C/stripes125A-C, as they are moved across the substrate 12 will be disclosed, inaccordance with one embodiment of the present invention. By way ofexample, the substrate 12 may be a photomask, reticle or wafer. Asmentioned, the beam sweep or swath, S, is in a direction such that,after passing through the inspection optics, it is directed parallel tothe Y-axis as viewed at the substrate 12. As the beams are swept, thestage (not shown in FIG. 8) carrying the substrate 12 under test iscaused to move back and forth in the direction of the X-axis, for aninspection length I, while being incremented in the Y-direction at theend of each traverse so that the scanning spots 124A-C are caused tosweep along a serpentine path 153 across a predetermined area of thesubstrate 12. The predetermined area may correspond to a singleidentified sub area, a plurality of identified substrate sub areas (suchas individual dice in the case of a photomask) or the entire substrate.

Prior to starting the inspection operation, the operator generallyaligns the substrate in the proper orientation and defines the area tobe inspected. The inspection area may be defined using the controlcomputer of FIG. 1. In FIG. 8, the inspection area corresponds to threedice of a photomask respectively designated 150A, 150B, and 150C. Inthis manner, the surface area of the dice 150A-C are swept in a seriesof contiguous swaths, S, by the scanning spots 124A-C. As shown, eachsweep in the X-direction produces an inspection area 155, which is theproduct of the swath, S, and the inspection length, I. The swath, S, maybe overlapped at each traverse (in the Y-direction) so as to ensure thatthe entire inspection area is scanned without missing any areastherebetween. Accordingly, the field size or swath is larger andtherefore more of the substrate is inspected for each scan, and as aresult there tends to be fewer turnarounds. By way of example, by usingthree beams the inspection speed is approximately tripled.

By way of example, in a die to die comparison mode, the inspection ofthe substrate 12 ordinarily starts at the upper left hand corner of thefirst die 150A and follows the serpentine pattern 153. As the stageslowly moves in the X direction, the laser beams rapidly sweep in theY-direction. In this manner, a swath S is scanned and the digitizedoutput of the detectors is stored in, for example, the electronicssubsystem 19 of FIG. 1. In the case of a transparent or partiallytransparent substrate, detection of the image is accomplished by thetransmission detectors 61. The light reflected from the substrate isdetected by a reflected light detectors 65. As such, when the scanreaches the right boundary of the care area of the first die 150A, imagedata derived from die 150A (left to right) is stored in subsystem 19.Correspondingly, when the scan reaches the right boundary of the seconddie 150B, image data derived from die 150A, which is now stored insubsystem 19, is compared with the data derived from die 150B. Anysubstantial difference is designated a defect. In a similar manner, thedata from die 150C is also compared with the data derived from die 150Bor 150A. When the scanning process reaches the right boundary of the die150C, the stage is moved in the Y-direction an amount slightly less thanthe swath width and the stage starts a return trace in the X-direction.In this manner the care areas of the dice are traversed by theserpentine motion.

Although only the die to die inspection mode is described, it should benoted that this is not a limitation and that a die-to-databaseinspection or simultaneous inspection may also be performed. The die todatabase inspection is similar to die-to-die inspection except that thedie is compared to a simulated image generated by the control system.

Furthermore, the system can be arranged for sampling the amplitude ofthe plurality of scan signals at precise intervals. The sampled scansignals combined with precise control of the stage motion and otherfactors are used to construct a virtual image of the pattern surface ofthe substrate. The sampling rate of each of the plurality of scans canbe varied to compensate for the different and non-linear scan rates ofthe individual scans caused by the diffraction grating. The system isalso capable of measuring the mentioned scan rate variations andautomatically calibrating the required sample rate corrections. Samplingis generally well known in the art and for the sake of brevity will notbe discussed in greater detail.

Referring now to FIGS. 9-13, the transmitted light optics 58 will bedescribed in greater detail. In brief, FIG. 9 is a side viewillustration of the transmitted light optics 58 and the transmittedlight detector arrangement 60. FIG. 10 is a detailed perspective diagramof the prism 100, in accordance with one embodiment of the presentinvention. FIG. 11 is a side view diagram, in cross section, of theprism 100, in accordance with one embodiment of the present invention.FIGS. 12A-C are side view diagrams illustrating the various positions ofthe first transmitted lens and spherical correction lens, in accordancewith one embodiment of the present invention. FIG. 13 is a flow diagramof the transmitted optics set-up, in accordance with one embodiment ofthe present invention.

Referring first to FIG. 9, the transmitted light optics 58 areconfigured to receive a plurality of transmitted light beams 94A-C andthe transmitted light detector arrangement 60 is configured to detectthe light intensity of each of the plurality of transmitted beams 94A-C.More particularly, the transmitted light optics 58 are configured toperform a variety of tasks associated with the transmitted beams 94A-Cincluding, but not limited to, collecting the transmitted light 94A-C,maintaining beam separation, and focussing the separated beams ontoindividual light detectors 61A-C of the transmitted light detectorarrangement 60. As mentioned, prior to receiving the transmitted lightbeams 94A-C, the focused light beams 78A-C produced by the inspectionoptics 54 are made incident on the surface of a transmissive substrate.At least a portion of the incident light beams 78A-C are transmittedthrough the substrate 12 as transmitted light beams 94A-C to thetransmitted light optics 58. The diagram is shown in the X & Zdirections such that the Y direction is coming out of (or into) thepage.

As shown in FIG. 9, the transmitted light optics 58 used to receive thetransmitted light beams 94A-C include a first transmitted lens 96, aspherical aberration correction lens 98, a prism 100, and a secondtransmitted lens 102. The first transmitted lens 96 collects thediverging transmitted light beams 94A-C and along with the sphericalaberration correction lens 98 focuses the transmitted light beams 94A-Conto the prism 100. The first transmitted lens 96 and the sphericalaberration correction lens 98 work as a unit to produce well-definedspots on the first faceted prism 100. Focussed and well-defined spots atthe prism 100 serve to reduce spot overlapping (i.e., beam intersectstwo facets), which tends to cause unwanted cross talk. As should beappreciated, cross talk may lead to erroneous signals, which may lead toinspection failure. In one embodiment, an automatic collection opticssubsystem 148 is used to maintain focus on the prism 100. The automaticcollection optics subsystem 148 maintains focus on the prism 100 bycontrolling the movement of the first transmitted lens 96, the sphericalaberration correction lens 98, and the prism 100. The automaticcollection optics subsystem 148 will be described in greater detailbelow. Furthermore, although not shown in FIG. 9, a third lens,positioned between the spherical aberration correction lens 98 and theprism 100, may be used help image quality at the prism 100.

To elaborate further, when the transmitted light beams 94A-C leave thesubstrate 12, they are bent by the first transmitted lens 96 towards theprism 100. For ease of discussion, the bent beams are designated94A′-C′. As the bent beams pass through the spherical aberration lens98, they are bent further, although not much further, by the sphericalaberration lens 98 towards the prism 100. As shown, the beams passthrough a pupil plane 130 while travelling between the lenses 96 and 98.Upon leaving the spherical aberration lens 98, each of the bent beams94A′-C′ are individually incident on an individual facet of the prism100. As should be appreciated, the prism 100 is generally located at animage plane because that is where the beams are distinct and isolatedfrom each other. If the prism is positioned anywhere else, the beams mayoverlap and cause errors in inspection. Furthermore, the prism 100correspondingly bends and separates the light beams, now designated94A″-C″, such that they are separately directed towards one of the threeindividual detectors 61A-C. As should be appreciated, the prism 100 isused to ensure that each of the beams goes to an individual detectorrather than all going to one detector.

In one embodiment, a prism system 99 having a plurality of prisms (shownhere as prism 100 and prism 101) is used to accommodate themagnification range produced by the telescope of the inspection optics.For example, at certain magnifications the scans can be very large, andtherefore a larger prism is needed to effectively separate the scanswithout cross talk. In one implementation, therefore, the prism 100 is ahigh resolution prism (e.g., small scan) and the prism 101 is a lowresolution prism (e.g., large scan). Each of the prisms 100 and 101 isarranged to move in and out of the optical path 120 so as to adjust fordifferent telescope magnifications.

Furthermore, both of the prisms 100 and 101 may be configured to moveout of the optical path 120 for the same reasons as the diffractiongrating 76, i.e., when only one beam is needed. As such, when the prismsare moved out of the optical path, a single beam is allowed to continuethrough the transmitted optics 58 without being separated. This may bedesired for alignment and set-up modes, which are used to align theoptics during an inspection set-up. This may also be desired for viewingthe substrate 12 in a live or review mode. The review mode is often usedto characterize a known defect.

Referring now to FIGS. 10 & 11, the prisms will be described in detail.Each prism includes three facets 160A, 160B, and 160C, whichrespectfully correspond to each of the transmitted light beams 94A-C(shown as spots on each of the facets). The first facet 160A correspondsto beam 94A; the second facet 160B corresponds to beam 94B; and thethird facet corresponds to beam 94C. Furthermore, during a beam sweepeach of the beams 94A-C forms a scan 95A-C in the Y direction on thesurface of their corresponding facets 160A-C. Although only three facetsare shown, it should be understood that the amount of facets correspondsto the amount of beams and therefore if more or less beams are used thenthe amount of facets will change accordingly.

To elaborate further, each of the facets are configured with a facetwidth 162, a facet length 163, and a facet angle 164. The facet widths162 are defined by facet edges 165. The facet widths 162 are configuredto provide a sufficient amount of tolerance 166 between the facet edges165 and the edges of the spots 94 in order to maintain proper beamseparation. As should be appreciated, if the tolerance 166 is too smallor too large, on either side of the spot, then the beams may drift overthe facet edge and into an adjacent facet or off of the facet alltogether. This type of drift may lead to alignment errors, out of focuserrors, and cross talk errors. In most cases, the facet widths areselected relative to the size of the spots (spot size is designated 167)being used to scan the substrate in order to maintain an appropriatetolerance. For instance, when a larger spot size is used, i.e., lowresolution, the facet width is arranged to be correspondingly larger,and when a smaller spot size is used, i.e., high resolution, the facetwidth is arranged to be correspondingly smaller. Furthermore, the facetlength 163 is configured to provide a sufficient distance for scanning.That is, the facet length 163 is arranged to be greater than the swathheight, S.

With regards to the facet angle 164, the facet angle 164 is arranged todirect the beams towards one of the detectors of the detectorarrangement. As should be appreciated, the focal length (shown as 168 inFIG. 10) of the second lens and the facet angle 164 work together to getthe desired beam separation at the detector plane. It is generallybelieved that for a given detector separation (shown as 170 in FIG. 9)and a given detector size (not shown), a larger facet angle, whichproduces a larger separation, needs a smaller focal length, andconversely, a smaller facet angle, which produces a smaller separation,needs a larger focal length. As such, when the facet angle is adjusted,so is the focal length of the lens.

Referring back to FIG. 9, after the separated light beams 94A″-C″ passthrough the prism 100, they pass through the second transmitted lens102, which is arranged for focussing the separated beams on thedetectors 61A-C of the transmitted detector arrangement 60. Upon leavingthe second transmitted lens 102, the beams are made incident on theindividual detectors 61A-C. As discussed, the second transmitted lens102 focuses each of the beams 94A, 94B, 94C on a single respectivedetector. For example, beam 94A is focused onto transmission detector61A; beam 94B is focused onto transmission detector 61B; and beam 94C isfocused onto transmission detector 61C. As should be appreciated, eachof the transmission detectors 61A, 61B, and 61C is arranged formeasuring the intensity of the transmitted light. By way of example, thethree detectors may be individually packaged chips or may be part of asingle chip package (as shown). It should be noted, however, that thesingle chip package has the advantage of allowing closer packeddetectors that allow for shallower incident angles.

Referring now to the collection subsystem 148, the collection subsystem148 is used to adjust the positions of the first transmitted lens 96,the spherical aberration correction lens 98, and the prism 100 in orderto properly focus and align the beams 94A-C on the prism 100. Asmentioned, if the beams 94A-C are not focussed and/or aligned on theprism 100, then the beams 94A-C may not achieve proper separation orthey may overlap one another. Improper separation and/or overlapping maycause erroneous inspections.

The collection subsystem 148 generally includes a system softwareelement 172, a control interface 174, an objective position sensor 176,a first transmitted lens drive 178, a spherical aberration correctionlens drive 180, and a prism drive 182. The system software element 172may be arranged to act as the master controller of the subsystem, andmay be implemented on a control computer such as the one described inFIG. 1. The system software element 172 is coupled to the controlinterface 174, which is configured to distribute and receive signals toand from the drives and sensors. The control interface 174 may beimplemented in an electronic subsystem such as the one described inFIG. 1. Furthermore, the control interface 174 is coupled to theobjective position sensor 176, the first transmitted drive 178, thespherical aberration correction lens drive 180, and the prism drive 182.

In brief, the objective position sensor 176 is arranged for monitoringthe position of the objective lens 90, for example, when the objectivelens 90 is moved along the optical axis to stay in focus with thesubstrate (Auto-Focus). The first transmitted drive 178 is arranged formoving the first transmitted 96 along the optical axis 120, and moreparticularly in the Z direction (as shown by arrow 190). The sphericalaberration correction lens drive 180 is arranged for moving thespherical aberration lens 98 along the optical axis 120, and moreparticularly in the Z direction (as shown by arrow 191). The prism drive182 is arranged for moving the prisms 100 and 101 orthogonal to theoptical axis 120, and more particularly in the X direction (as shown byarrow 192). By way of example, all of the drives may include a motorizedlinear actuator for driving the movements of the aforementionedelements. Motorized actuators are well known in the art and for the sakeof brevity will not be discussed in detail.

There are typically several conditions involved in focussing andalignment. A first condition includes setting a nominal position for thefirst transmitted lens 96, the spherical aberration correction lens 98,and the prism 100 for a predetermined substrate thickness and lightmagnification (e.g., telescope). As should be appreciated, differentsubstrate thickness', as well as different magnifications, requiredifferent optical positions because of how they effect the transmittedlight. For example, for a given optical position, i.e., the opticsremain stationary, a change in substrate thickness (for example, from a¼ in. substrate to a 90 mil substrate) tends to shift the position ofthe image plane and therefore the light at the prism tends to be out offocus. In addition, for a given optical position, a change inmagnification tends to cause a change in beam separation and placementand therefore the light at the prism tends to overlap.

Accordingly, once the substrate size and magnification level isdetermined, the collection subsystem 148 moves the first transmittedlens 96, the spherical aberration correction lens 98, and one of theprisms 100, 101 to a pre-determined nominal value. In effect, thesoftware element 172 via a control computer sends position informationto the control interface 174, which then sends a movement signal to thefirst transmitted lens drive 178, the spherical aberration correctionlens drive 180 and the prism drive 182 to move their respective opticsaccording to the pre-determined nominal values. In general, for thickersubstrates the lenses are moved away from the patterned surface of thesubstrate, while for thinner substrates the lenses are moved closer tothe pattern surface of the substrate. Although they both move, it shouldbe noted that the spherical aberration lens moves a larger distance thanthe first transmitted lens. Additionally, for larger spot sizes (orlarge pixel sizes), a larger prism, i.e., prism 101, is moved into theoptical path, and for smaller spot sizes (or small pixel sizes), asmaller prism, i.e., prism 100, is moved into the optical path.

FIGS. 12A-C are exemplary diagrams illustrating the nominal positionsfor various substrate thickness.′ In these figures, the thickness of thesubstrate is designated 180, the distance between the first transmittedlens 96 and the patterned surface 11 of the substrate 12 is designated182, and the distance between the first transmitted lens 96 and thespherical aberration correction lens 98 is designated 184. As shown, thedistances 182 and 184 are directly proportional to the thickness 180 ofthe substrate. That is, the distances 182 and 184 are smaller for thinsubstrates (as shown in FIG. 12C) and larger for thick substrates (asshown in FIG. 12A). Also as shown, the lower collection lens is moved asmall distance relative to the distance moved by the sphericalaberration lens.

Furthermore, and referring back to FIG. 9, a second condition includescalibrating the positions of the first transmitted lens 96 and the prism100. Calibrating is a fine tuning process that generally aligns andfocuses the optics. Calibration steps are typically conducted on everysubstrate to be inspected, and sometimes these steps are periodicallyrepeated during the inspection process for long inspection runs. Withrespect to the prism 100, the prism is calibrated to find the bestposition of the prism orthogonal to the optical axis 120. For example,the prism 100 may be calibrated to center the incident beams on each ofthe facets 160A-C (as shown by the center to center distance 169 in FIG.11). With respect to the first transmitted lens 96, the lens iscalibrated to find the best position of the lens along the optical axis120. For example, the lens position may be calibrated so as to focus theincident beams on each of the facets 160A-C. If the prism 100 and thefirst transmitted lens 96 are not aligned and focused properly, thenthere may be very little tolerance for the incident beams duringinspection. As should be appreciated, a small tolerance may lead tolight beam cross talk or improper beam separation. By way of example,thermal changes may effect the position of the lenses and as a resultthe beams may begin to cross over to an adjacent facet. In some cases,the calibration step includes measuring the amount of cross talk at thetransmitted light detectors and moving the first transmitted lens andprism relative to their preset nominal values so as to minimize theamount of cross talk. The movement of the lens and prism are implementedthrough the control interface 174, which sends a signal to theirrespective drives to move accordingly. In most cases, these calibrationsare done automatically by the control system 17, and as a result, noactions, instructions or information are required for the operatorperforming the inspection.

Also, the pattern surface may be slightly sloped or tilted, and,therefore, a third condition includes tracking the position of the firsttransmitted lens 96 relative to the position of the patterned surface 11of the substrate 12 so as to maintain focus throughout the inspection.There are many ways to implement tracking. In one embodiment, trackingis implemented by moving the first transmitted lens 96 relative to theobjective lens 90. As discussed above, during an inspection, theobjective lens 90 is arranged to stay in focus with the substrate 12 bymoving up and down along the optical axis 120 (in the Z direction).Accordingly, the objective lens movement can be used to also track thefirst transmitted lens 96. In this embodiment, a distance C between thefirst transmitted lens 96 and the objective lens 90 is heldapproximately constant. As such, when the objective lens 90 moves afocus distance z, for example, the first transmitted lens also moves adistance z. The distance C is based on the calibrated position of thefirst transmitted lens 96. In order to implement tracking, the objectiveposition sensor 176 detects the position of the objective lens 90, andcorrespondingly sends a signal to the control interface 174, which thensends a signal to the first transmitted lens drive 178 to move the firsttransmitted lens 96 accordingly. Although only one embodiment oftracking is described, it should be noted that this is not a limitationand that other tracking methods may be used. For example, the firsttransmitted lens could also track the surface of the substrate directlywithout using the objective lens as a reference.

To elaborate further, FIG. 13 is a flow diagram of an inspection set-upprocedure 200, in accordance with one embodiment of the invention. Inoperation 202, the substrate is initially placed into the inspectionsystem, for example, the inspection system 10 as shown in FIG. 1. Inmost cases, this operation includes positioning the substrate on thestage 16. After the substrate is placed in the system and on the stage(operation 202), the thickness of the substrate is determined inoperation 204. Determining the thickness of the substrate is generallywell known in the art and for the sake of brevity will not be discussedin detail. After or during operation 204, the desired magnificationlevel for the telescope is set in operation 206. The magnification levelis generally chosen by the operator implementing the set-up. If theoperator desires greater sensitivity then a high magnification level istypically chosen. Conversely, if the operator desires greater scanningspeeds then a low magnification level is typically chosen.

Following operation 206, the position of the transmitted light opticsare set at nominal values in operation 208. As mentioned, thetransmitted optics with nominal values include the first transmittedlens 96, the spherical aberration correction lens 98 and the prism 100.The nominal values are generally selected by the system software andgenerally depend on the thickness of the substrate and the magnificationlevel of the telescope. In most cases, the system software knows thestate of the thickness and magnification and therefore it signals theinterface to move the optics accordingly. By way of example, if thesystem software selected a program for highest magnification, and a ¼″substrate, then the first transmitted lens 96 and the sphericalaberration correction lens 98 would move to a nominal ¼″ substrateposition. The appropriate prism is then moved to a nominal position forthe high magnification inspection.

After the nominal values are set in operation 208, the transmitted lightoptics are calibrated in operation 210. The transmitted light opticsthat are calibrated include the first transmitted lens 96 and the prism100. The calibration sequence generally includes collecting light valuesat the detectors 61A-C, analyzing the collected light values at thecontrol interface 174, calculating a new target position at the controlinterface 174, and adjusting the position of the optics, e.g., firsttransmitted lens 96 and/or the prism 100. In most cases, this sequenceis repeated until the minimum cross talk positions for the substrate tobe inspected are found. Collecting light values allows alignment to bebased directly on cross talk, rather than on secondary factors such asmeasuring the exact substrate thickness, etc.

After calibration, the process flow proceeds to operation 212 whereinspection of the substrate begins. That is, the system begins to scanthe substrate. As it scans, the objective lens movement is tracked sothat the first transmitted lens can maintain a specific distance betweenthem. In general terms, the objective position sensor triggers the firsttransmitted lens to move. For the most part, only the first transmittedlens moves during inspection. During or after operation 212, the processflow proceeds to operation 214 where a decision is made to re-calibrate(yes) or end the scan (no). If the decision is to re-calibrate then theprocess flow proceeds back to step 210. If the decision is to endscanning then the process flow proceeds to step 216, which signifiesthat the scan is done.

Referring now to FIG. 14, the reflected light optics 62 will bedescribed in greater detail. The reflected light optics 62 areconfigured to receive a plurality of reflected light beams 92A-C and todirect the received beams to the reflected light detector arrangement64. The reflected light detector arrangement 64 is configured to detectthe light intensity of each of the plurality of reflected beams 92A-C.More particularly, the reflected light optics 62 are configured toperform a variety of tasks associated with the reflected beamsincluding, but not limited to, collecting the reflected light,maintaining beam separation, and focussing the separated beams ontoindividual light detectors of the reflected light detector arrangement.As mentioned, prior to receiving the reflected light beams, the focusedlight beams produced by the inspection optics are made incident on thesurface of a reflective substrate and as a result the light beams arereflected by the substrate to the reflected light optics 62. The diagramis shown in the X & Z directions such that the Y direction is coming outof (or into) the page.

Although the reflected optics are similar to the transmitted lightoptics, the reflected light optics are not nearly as complex. Thetransmitted optics 58 are a dynamic system, while the reflected optics62 are a static system. This is generally related to the fact that thebeams are reflected off the surface of the substrate, rather than beingtransmitted through the substrate, i.e., no plate thickness tocompensate for, and that the reflected beams pass back through theobjective lens, i.e., no focussing to consider because the objective isalways in focus. Furthermore, only one prism is needed in the reflectedoptics because the reflected beams also pass back through the telescopeand therefore the spot size is the same with reflected light, i.e., themagnified beam is de-magnified.

As shown in FIG. 14, the reflected light optics 62 used to receive thereflected light beams 92A-C include a portion of the inspection optics(e.g., objective lens 90, telescope 88), a quarter wave plate 104, abeam splitter cube 82, a first reflected lens 108, a second facetedprism 110, and a second reflected lens 112. The objective lens 90 isarranged to collect the diverging reflected light beams 92A-C after theyreflect off the surface 11 of the substrate 12. Correspondingly, whenthe reflected light beams 92A-C leave the objective lens 90, they passthrough the quarter wave plate 104, as they approach the telescope 88.The quarter wave plate 104 is arranged to alter the reflected light suchthat when the reflected light intersects the beam splitter cube 82 it issplit away from the path 84. Upon leaving the quarter wave plate 104,the reflected light beams 92A-C pass through a pupil plane 130 and thenthe telescope 88, which reduces the size of the beams. When the beams92A, 92B, and 92C leave the telescope 88, they are incident on the beamcube splitter 82. The beam splitter 82 is arranged to work with thequarter wave-plate 104 to direct the beams 92A, 92B, and 92C onto thepath 106. As shown, the beam splitter cube 82 is positioned between thetelescope 88 and the diffraction grating 76.

The beams 92A, 92B, and 92C continuing on path 106 are directed to thefirst reflected lens 108. As shown, the beams pass through a pupil plane130 before reaching the first reflected lens 108. The first reflectedlens 108 collects and focuses the beams, now designated 92A′, 92B′, and92C′ on the prism 110 (similar to the prism shown in FIGS. 10 & 11).Generally, only one lens is needed in the reflected optics, as opposedto the transmitted light optics, because the beams pass through thetelescope twice (magnify/de-magnify) and therefore the spot size tendsto be the same for reflected light. Similarly to the first transmittedlens 96, the first reflected lens 108 is set-up to produce well-definedspots on the prism 110. Again, the prism 110 is typically located at animage plane because that is where the beams are distinct and isolatedfrom each other.

Furthermore, the prism 110 correspondingly bends and separates the lightbeams, now designated 92A″-C″ such that they are separately directedtowards one of the three individual detectors 65A-C. As should beappreciated, the prism 110 is used to ensure that each of the beams goesto an individual detector rather than all going to one detector. Afterthe separated light beams 92A″-C″ pass through the prism 110, they passthrough the second reflected lens 112, which is arranged for focussingthe separated beams onto the detectors 65A-C of the transmitted detectorarrangement 64. Upon leaving the second reflected lens 112, the beams92A″-C″ are made incident on the individual detectors 64A-C. Asdiscussed, the second reflected lens 112 focuses each of the beams 92A″,92B″, 92C″ on a single respective detector. For example, beam 92A″ isfocused onto transmission detector 65A, beam 92B″ is focused ontotransmission detector 65B, and beam 92C″ is focused onto transmissiondetector 65C. As should be appreciated, each of the transmissiondetectors 65A, 65B, or 65C is arranged for measuring the intensity ofthe reflected light. Although not described in FIG. 14, it should benoted that the prism and detectors are similar to the prism anddetectors described in FIGS. 9-11.

As can be seen from the foregoing, the present invention offers numerousadvantages over the prior art. Different embodiments or implementationsmay have one or more of the following advantages. One advantage of thepresent invention is that faster scanning speeds can be achieved to agreater degree than possible in the prior art. By way of example, mostexisting inspection systems use a single beam to scan the surface of asubstrate. In contrast, the present invention uses three beams to scanthe surface of the substrate. The three beams produce a scanning spotdistribution with about three times the scanning swath of a single beam.As such, the three beam system inspects more of the substrate per swathand is therefore about three times as fast as the single beam system.Another advantage of the present invention is that three beams areproduced without the added cost of using multiple lasers, objectives,etc.

Another advantage of the present invention is that the scanningdistribution provides scanning spots, which are spatially separated fromone another. Spatially separating the spots ensures that the transmittedlight and/or the reflected light is received at separate detectors.Another advantage of the present invention is that the system isscalable to accommodate more beams. Another advantage of the presentinvention is that it enables faster inspection using existing scannersand detectors, rather than using higher speed scanners and detectorsthat tend to have technical limitations.

While this invention has been described in terms of several preferredembodiments, there are alterations, permutations, and equivalents, whichfall within the scope of this invention. For example, in an alternateembodiment, a beam splitter cube may be used to separate the singlelight beam into a plurality of light beams, and a mirror system may beused to direct the separated beams onto an optical path for scanning thesurface of a substrate. FIG. 15 shows one implementation of thisembodiment. In FIG. 15, the single beam 66 is made incident on a beamsplitter cube 300, which divides the single beam 66 into beams 302 and304. The beams 302 and 304 are then made incident on mirrors 306 and308, respectively, which redirect each of the beams, now designated 302′and 304′, to a new optical path. Although only two beams are shown,additional beam splitter cubes could be used to produce more beams. FIG.16 shows another implementation of the aforementioned embodiment. InFIG. 16, the single beam 66 is made incident on a beam splitter cube310, which divides the single beam into beams 312 and 314. The beams 312and 314 are then made incident on mirrors 316 and 318, respectively,which redirect each of the beams, now designated 312′ and 314′, backtowards the beam splitter cube 310. Before the redirected beams 316′ and318′ reach the beam splitter cube 310, however, they each pass through aquarter wave plate 320 that works with the beam splitter cube 310 toredirect each of the beams, now designated 312″ and 314″, onto a newoptical path.

Furthermore, it should also be noted that there are many alternativeways of implementing the methods and apparatuses of the presentinvention. By way of example, the techniques described could also beused in systems that inspect optical disks, magnetic disks and opticalsubstrates. It is therefore intended that the following appended claimsbe interpreted as including all such alterations, permutations, andequivalents as fall within the true spirit and scope of the presentinvention.

1. An optical inspection system for inspecting a surface of a mask,reticle or semiconductor wafer for defects, comprising: a light sourcefor emitting an incident light beam along an optical axis; an opticalsystem disposed along the optical axis and including one or more opticalcomponents for separating the incident light beam into a plurality ofindividual light beams, all of the individual light beams beingdedicated to defect inspection, all of the individual light beamsworking together to increase the speed of defect inspection, all of theindividual light beams forming scanning spots at different locations onthe surface of the mask, reticle or semiconductor wafer, the scanningspots being configured to scan the surface of the mask, reticle orsemiconductor wafer in order to find defects associated with the surfaceof the mask, reticle or semiconductor wafer when the mask, reticle orsemiconductor wafer and the plurality of individual light beams arelinearly moved relative to one another in a first direction, wherein theone or more optical components are further arranged to deflect theindividual light beams along individual scan paths in a second directionthat differs from the first direction, the individual scan paths beingoffset from each other in the first direction.
 2. The optical inspectionsystem as recited in claim 1 further comprising a light detectorarrangement including individual light detectors that correspond toindividual ones of a plurality of reflected or transmitted light beamscaused by the intersection of the individual light beams with thesurface of the reticle mask, or semiconductor wafer, the light detectorsbeing arranged for sensing the light intensity of either the reflectedor transmitted light.
 3. The optical inspection system as recited inclaim 2 wherein the light detector arrangement includes individual lightdetectors that correspond to individual ones of a plurality of reflectedand transmitted light beams caused by the intersection of the individuallight beams with the surface of the reticle mask, or semiconductorwafer, the individual light detectors being arranged for sensingcharacteristics of the reflected and transmitted light so as to performdefect analysis associated with the surface of the mask, reticle orsemiconductor wafer.
 4. The optical inspection system as recited inclaim 2 wherein the light detector arrangement includes individual lightdetectors that correspond to individual ones of a plurality of reflectedlight beams caused by the intersection of the individual light beamswith the surface of the reticle mask, or semiconductor wafer, theindividual light detectors being arranged for sensing characteristics ofthe reflected light so as to perform defect analysis associated with thesurface of the mask, reticle or semiconductor wafer.
 5. The opticalinspection system as recited in claim 2 wherein the light detectorarrangement includes individual light detectors that correspond toindividual ones of a plurality of transmitted light beams caused by theintersection of the individual light beams with the surface of thereticle mask, or semiconductor wafer, the individual light detectorsbeing arranged for sensing characteristics of the transmitted light soas to perform defect analysis associated with the surface of the mask,reticle or semiconductor wafer.
 6. The optical inspection system asrecited in claim 1 further comprising: a stage system for transportingthe mask, reticle or semiconductor wafer along the first direction. 7.The optical inspection system as recited in claim 1 wherein the opticalcomponents are further arranged to partially overlap the scan paths ofthe individual light beams in the second direction.
 8. The opticalinspection system as recited in claim 1 wherein the plurality ofindividual light beams all pass through a single aperture.
 9. An opticalinspection system for inspecting a surface of a mask, reticle orsemiconductor wafer for defects, comprising: a light source for emittingan incident light beam along an optical axis; a first set of opticalelements arranged for separating the incident light beam into aplurality of individual light beams dedicated to defect inspection,directing the individual light beams to intersect with the surface ofthe mask, reticle or semiconductor wafer, focusing the individual lightbeams to a plurality of scanning spots on the surface of the mask,reticle or semiconductor wafer, and scanning the scanning spots over thesurface of the mask, reticle or semiconductor wafer when the mask,reticle or semiconductor wafer and the plurality of individual lightbeams are linearly moved relative to one another in a first direction,wherein the first set of optical components are further arranged todeflect the individual light beams along individual scan paths in asecond direction that differs from the first direction, the individualscan paths being offset from each other in the first direction; and alight detector arrangement including individual light detectors thatcorrespond to individual ones of a plurality of reflected or transmittedlight beams caused by the intersection of the individual light beamswith the surface of the mask, reticle or semiconductor wafer, the lightdetectors being arranged for sensing the light intensity of either thereflected or transmitted light.
 10. The optical inspection system asrecited in claim 9 wherein the first set of optical elements is arrangedfor separating the incident light beam into the individual light beamsso that such individual light beams take the form of a plurality ofspatially distinct light beams, which are offset and staggered relativeto one another in the first direction and the second direction.
 11. Theoptical inspection system as recited in claim 10 further comprising asecond set of optical elements adapted for collecting either a pluralityof reflected light beams or a plurality of transmitted light beamscaused by the intersection of the plurality of individual light beamswith the surface of the mask, reticle or semiconductor wafer, and fordirecting individual ones of the collected light beams to individuallight detectors of the light detector arrangement.
 12. The opticalinspection system as recited in claim 9 wherein the first set of opticalelements comprises a beam deflector disposed along the first opticalaxis, the beam deflector being arranged for deflecting the light beamsuch that the individual light beams are caused to sweep across thesurface of the mask, reticle or semiconductor wafer in substantially thesecond direction from a first point to a second point.
 13. The opticalinspection system as recited in claim 12 wherein the beam deflectorcomprises an acousto-optic device for causing the light beam to bedeflected over a relatively small angle, the angle being at least one ofthe factors for determining a length of each of the individual scanpaths of each of the individual light beams.
 14. The optical inspectionsystem as recited in claim 9 wherein the first set of optical elementscomprises a beam separator disposed along the first optical axis, thebeam separator being arranged for separating the light beam into theplurality of individual light beams.
 15. The optical inspection systemas recited in claim 14 wherein the beam separator comprises a beamsplitter cube.
 16. The optical inspection system as recited in claim 14wherein the beam separator is a diffraction grating.
 17. The opticalinspection system as recited in claim 16 wherein the diffraction gratingis arranged for separating the light beam into the individual lightbeams so that such individual light beams takes the form of a pluralityof spatially distinct light beams, which when focused on the surface ofthe mask, reticle or semiconductor wafer produce a plurality of scanningspots which are offset and staggered relative to one another in thefirst direction and in the second direction, and which cause a portionof the scan length of the scanning spots to overlap one another in thefirst direction.
 18. The optical inspection system as recited in claim17 wherein the diffraction grating has a grating spacing and a gratingrotation about the optical axis, and wherein each of the scanning spotshas a specified overlap and separation that is controlled by the gratingspacing and the grating rotation.
 19. The optical inspection system asrecited in claim 16 wherein the diffraction grating is selected from oneof a transmission type grating or a reflective type grating.
 20. Theoptical inspection system as recited in claim 19 wherein thetransmission type grating is selected from one of a phase grating or anamplitude grating.
 21. The optical inspection system as recited in claim9 further comprising a stage for carrying the mask, reticle orsemiconductor wafer and for moving the mask, reticle or semiconductorwafer in at least two linear directions within an inspection plane.